Wednesday, June 16, 2010

Paint Mixing - High Thixotropic


Today I have a story about a major US cabinet manufacturer who was trying to measure and control the viscosity of their paint, which was being piped out and sprayed onto cabinets.

They had 55 gallon drums with a constant rpm mixer. The mixer was set at a speed so that when the drum was mostly empty, the ink would not foam. When they refilled the drum with new paint (at the same % solvent as what was in the drum) the mixer speed was not changed.

What is wrong here ? What do you think?

Hint: mixing, mixing, mixing.

When the drum was full there was less net mixing energy being applied to the paint and thus its viscosity (by efflux cup) increased. even though it was at the same % solvent.

Thus when automatic viscosity controls were attempted, they would over-dilute. The solution would have been to keep the levels more mid-range and provide more vigorous mixing, plus to only add small amounts of paint to the system. This would give you stability and then you could control viscosity.

With Thixotropic solutions you need to mix them until they reach a point of viscosity where a change in mixing energy does not result in a change of viscosity.